This invention relates to a method of milling a cerium-rich material for oxygen storage and oxygen release in an exhaust gas catalyst.
The need to lower vehicle exhaust emissions continues to create challenges, especially in the wake of stringent environmental regulations. The need to lower cold-start emissions is at the heart of most emissions reduction strategy, since cold-start emissions account for a significant portion of exhaust emissions from any given vehicle. During startup, hydrocarbons can be passed through the exhaust system before the catalyst can heat up and convert the hydrocarbons to more desirable gases. Although a large portion of hydrocarbons are reduced, an amount of hydrocarbons and other undesirable gases may be allowed to pass through the exhaust system without reduction thereof.
One solution to the cold-start emission problem is providing a material that is able to give off oxygen to the catalyst during startup conditions such that the xe2x80x9clight-offxe2x80x9d temperature of the catalyst is accelerated. The light-off temperature is the temperature at which the catalyst reacts with hydrocarbons and other exhaust gases to reduce these gases, which are undesirable, to more desirable gases such as carbon dioxide. Oxygen, when fed to the catalyst, creates an exothermic reaction to the catalyst, resulting in increased temperature which allows the catalyst to reach its light-off temperature more quickly.
Cerium has long been used as a component of automotive catalysts for oxygen storage and other functions. In catalysts, ceria is used to rapidly switch between CeO2 and CeO2xe2x88x92x under lean and rich conditions,.respectively, which allows for the capacity to rapidly store and release oxygen. In general, the cerias utilized have relatively high purity resulting from essentially complete refining and separation of the lanthanide ores into their component oxides. As a result of the refining processes associated, such high purity ceria is relatively expensive. In fact, the cost of cerium accounts for a substantial portion of the non-precious metal material cost of a catalyst.
Currently, cerium-zirconium oxide mixtures are being used for oxygen storage and release in exhaust systems. Cerium-zirconium mixed oxides have much better performance properties than high purity cerium oxide, including enhanced thermal, redox, and catalytic properties, and have thus supplanted the use of pure ceria. In pure ceria, the conversion between CeO2 and CeO2xe2x88x92x is strongly dependent on surface redox phenomena. A reduction of the surface area thus has a negative impact on the capacity to rapidly store and release oxygen. The introduction of ZrO2 into the CeO2 lattice is believed to strongly enhance the reduction features of ceria. Among other things, the introduction of zirconium enhances the formation of structural defects which contribute to the enhanced redox behavior.
A number of studies have been performed to research ways oxygen storage and release may be provided in a more cost-effective manner. One way to potentially reduce costs is to use low-grade, less-refined ceria rather than highly refined ceria. Low grade ceria can be recovered in the refining process after the roasted ore, typically bastnasite, is split, by hydrochloric acid leaching, into an insoluble fraction known as cerium concentrate and a soluble fraction known as lanthanum concentrate. Cerium concentrate typically is about 60% ceria by weight and represents the least pure form of ceria that may be considered for catalytic applications.
However, low grade rare-earth concentrates have historically exhibited relatively poor oxygen storage capabilities. Without any modification thereto, low grade rare-earth concentrates show little promise as a catalytic material.
The present invention prepares the cerium concentrate such that its oxygen storage and release properties are enhanced, and thus provide a cost-effective catalytic material for providing oxygen.
It is an object of the invention to provide a method of making a cerium-rich material that is inexpensive and which transforms low grade ceria into a material that is effective in storing and releasing oxygen for exhaust gas systems.
The method includes providing a cerium-rich material having oxides with about 60 wt. % ceria and oxides of about 40 wt. % lanthana and neodymia, and using high energy vibratory mechanical milling at a predetermined oscillation frequency and amplitude for preparing the mixed oxides. The milling continues until the cerium-rich material is at a substantially uniform molecular mixture of ceria, lanthana, and neodymia, whereby the milled cerium-rich material exhibits enhanced ability of storing and releasing oxygen.